It seems the forum resizes the pics, click on copy and paste the link for full size.
Max forum picture dimensions are 640px in either direction. In the FAQ, we ask people NOT to post larger photos, but to resize them to 640 first and link them to a larger one if required.
Yes, 640 is small but we did have a good think about this and it was after the days of dial-up, before anyone asks. 1024 doesn't work well on phone screens but the main issue is that, with tens of photos on a gallery page being pulled from numerous different servers, it gets slow even on broadband and, on mobile, it eats into data allowances very fast if they're all 3MB.
I agree some people are too lazy and upload huge pics straight from their camera unedited but I reduce to 1600x1200 (average about 400kb), or smaller like 1200x900. For pics where there's no need to zoom in to look at details, I'll go even smaller.
I thought the forum just shows them smaller, without actually reducing the dimensions and file size.
Right click and "view image info" on a pic in this thread gives something like:
Size: 426.8 KB
Dimensions: 1,600px × 1,200px (scaled to 640px × 480px)
Doesn't the pic come from the image host (eg Imgur) direct to your device?
Also it seems you can see the pics in a bigger size up to the original uploaded size by zooming in the web page (ctrl +), certainly bigger than 640x across, the text will get bigger as well.